1. Field of the Invention
The present invention relates to an automatic focusing apparatus which is suitably used in video equipment comprising image pickup means such as a video camera, and performs automatic focusing control using an image pickup signal output from the image pickup means.
2. Related Background Art
Video equipment comprising image pickup means such as a video camera adopts an automatic focusing (AF) system wherein a predetermined signal component, which changes in correspondence with a focusing state, is extracted from an image pickup signal output from the image pickup means, and the extracted signal component is used as an evaluation signal of the focusing state.
For example, an AF apparatus based on a so-called "hill-climbing AF system" is known. In the hill-climbing AF system, a high-frequency component indicating sharpness of an image is extracted from an image pickup signal output from an image pickup element such as a CCD, and a focusing lens is driven to maximize the level of the extracted high-frequency component, thus achieving AF control.
FIG. 1 shows the arrangement of a conventional AF apparatus based on the "hill-climbing system".
Referring to FIG. 1, a focusing lens 1 is moved by a lens driving motor 57 in the optical axis direction to achieve a focusing operation. Light transmitted through the focusing lens 1 forms an image on the image pickup surface of an image pickup element 3, the formed image is photoelectrically converted into an electrical image pickup signal, and the image pickup signal is output. The image pickup signal is sampled and held by a CDS (double correlation sampling circuit)/AGC (auto-gain control circuit) 4, and is amplified to a predetermined level. Thereafter, the image pickup signal is converted into a digital image pickup signal by an A/D converter 5. The digital image pickup signal is input to a process circuit of a camera, and is converted into a standard television signal based on the NTSC system. The digital image pickup signal is also input to a band-pass filter (to be referred to as a BPF hereinafter) 100.
The BPF 100 extracts a high-frequency component from the image pickup signal, and a gate circuit 101 extracts only a signal corresponding to a portion set to be an in-focus detection region in a photographing field. A peak hold circuit 102 holds a peak value of the output from the gate circuit 101 at an interval synchronous with an integer multiple of a vertical synchronization signal. Since this peak hold value is utilized in AF control, it will be referred to as an AF evaluation value hereinafter.
A speed discrimination circuit 104 sets a focusing speed corresponding to an in-focus degree on the basis of the AP evaluation value. More specifically, the circuit 104 instructs a motor driver 56 to vary the motor speed, so that the motor speed is increased in a considerably out-of-focus state, and the motor speed is decreased in a slightly out-of-focus state. On the other hand, a direction discrimination circuit 103 sets the motor driving direction in a direction to increase the AF evaluation value, thereby increasing the in-focus degree. Such control is the above-mentioned hill-climbing control.
In the conventional AF apparatus based on the hill-climbing system, since AF control is performed using only a focus detection signal extracted from only one focus detection region set in the photographing field, the AF operation becomes unstable depending on objects and photographing conditions.
In order to realize the above-mentioned AF apparatus using the image pickup signal, many means such as means for extracting an evaluation signal, which changes in correspondence with a focusing state, from an image pickup signal, means for setting a detection region in a photographing field for obtaining the evaluation signal, arithmetic & control means for executing a control algorithm for driving a focusing lens on the basis of the evaluation signal, means for controlling the operation timing with other circuits other than those associated with an AF function, and the like are required. For this reason, the circuit arrangement, and various setting and adjustment operations as a whole are complicated. Also, use of both analog and digital circuits disturbs efficient signal processing, and undesirably allows easy mixing of noise. Thus, the AF apparatus suffers from many problems including the above-mentioned problems, and it is difficult to realize a stable, efficient, and high-precision AF apparatus.
In particular, in a focus detection apparatus using an image pickup signal, since a dynamic image is to be picked up, a detection region corresponding to a change in image must be set, and a focus detection algorithm corresponding to a change in image must be executed. However, it is very difficult to realize these operations by the above-mentioned circuit arrangement. Therefore, demand has arisen for practical means which can efficiently and stably control circuit operations, can simplify the circuit arrangement, and allows easy connections with other control circuits to improve efficiency of the entire system.